The knee joint is the largest and most complex joint in the human body. Owing to its anatomical structure it is susceptible to injury and damage through wear and tear. The menisci reside within this joint and play a vital role in joint stability, shock absorption and load transmission. Hence, injury to the menisci leads to altered joint biomechanics, and coupled with an inability to heal due to its avascularity, this leads in turn leads to subsequent degeneration of surrounding tissues characteristic of osteoarthritis (OA). There are approximately one million procedures related to the meniscus every year in the United States alone, and current treatment options only delay the onset of OA and do not provide a cure. Therefore, a meniscal replacement is necessary in order to halt the progression of OA and restore native joint biomechanics. Treatment depends on the severity of a tear and the age and health status of the patient, typical treatments include a total meniscectomy where the entire meniscus is removed, a partial meniscectomy where the surgeon removes as little of the meniscus as possible and any unstable meniscal fragments are removed with the remaining meniscus edges smoothed so that there are no frayed ends. Alternatively the tear can be repaired by sutures or the like, however not all tears can be repaired in this way and even if they can this method can cause injury to the surrounding tissue or create inferior fibrous scar tissue formed in outer tears that heal. A yet further option is that the meniscus can be totally replaced by, for example, a collagen meniscal implant composed of collagen I or some other unorganized matrix or by a knee joint prosthesis. Many avenues have been investigated in the search for a meniscal replacement, from autogenous, allogeneic and xenogeneic tissue sources, to more recent tissue engineering strategies. A limitation of allografts is their tissue availability, potential for disease transmission, their overall tissue quality and the possibility of tissue rejection. A limitation of xenogeneic tissue is how to achieve biocompatibility and immunologically inert tissue.
The meniscus comprises four different and distinct cell populations; (i) endothelial cells reside in the vasculature (ii) fusiform cells populate the superficial region; (iii) rounded cells are found in the hyaline-like region and; (iv) fibroblast-like cells are found deep within the central region, the presence of deeply embedded cells is one of the reasons why historically decellularisation of allogenic and xenogeneic tissue sources has proven difficult to achieve. WO 2008/059244 describes a method of decellularising meniscal tissue.
Ligaments connect bones to other bones to form a joint, some ligaments limit the mobility of articulations, or prevent certain movements altogether. Ligaments are mechano-responsive, mechanically strong and viscoelastic. Ligaments gradually lengthen when under tension, and return to their original shape when the tension is removed. However, they cannot retain their original shape when stretched past a certain point or for a prolonged period of time. This is one reason why dislocated joints must be set as quickly as possible: if the ligaments lengthen too much, then the joint will be weakened, becoming prone to future dislocations. Ligament injuries are relatively common both in man and animals for example in 2005 a study estimated that $1.32 billion was spent in the United States in treating the cranial cruciate ligament of dogs. In humans, ligament injuries are more common in the knee joint which frequently affects the cruciate ligaments. Ligament injuries can be divided into two types, acute and chronic injury. Ruptures are the most common acute form and often occur in sports settings. The complete division or detachment of a ligament causes immediate loss of its function, which is permanent unless it is repaired. Chronic ligament injury is manifested by pain and swelling. Ligaments are soft connective tissue that are composed of: closely packed parallel collagen fibre bundles; ligament fibroblasts which are the dominant cells responsible for ligament homeostasis and repair and; stem cells which play a vital role in maintenance and repair. Therefore, it is essential in any decellularisation process that the histo-architecture and biomechanical properties of these complex tissues are preserved.
The composition of bone matrix is approximately one third organic and two thirds inorganic matter. The organic matter, synthesized by the osteoblasts is collagen and proteins like proteoglycans and glycoproteins. Other cells present in bone are osteocytes, osteoclastsosteoprogenitors and bone lining cells. The other inorganic matter is mostly crystallized calcium phosphate salts and calcium carbonate, and a few other minerals. Bone matrix is a composite which means it has characteristics of the hard, strong inorganic matter and some flexibility and give from the collagen.
A major problem associated with any implants comprising a composite or mixture of different natural biological materials, each having different properties and functions, is that one particular decellularisation treatment may be effective for one tissue type of the composite/mixture but may be deleterious to another tissue type resulting in an impaired overall composite product that is not fit for purpose. The problem is compounded according to the number of different tissue types in the composite implant. For example, since bone itself is a composite material any implant that comprises bone plus another tissue type, for example ligament, meniscus and/or enthesis (a cartilage-like tissue found where the meniscus or ligament attaches to the bone) is effectively three or more different tissue types each requiring specific decellularisation treatments that do not impair or alter the characteristics of any of the other tissue types.
Osteochondral lesions can cause pain and a lack of motion in joints but also often result in increased cartilage wear and degradation of the synovial joint to an osteoarthritic state. A number of surgical interventions can be employed to repair initial cartilage damage and prevent disease progression to osteoarthritis. These treatments, however are often ineffective, providing a mechanically inferior repair material, lack integration with surrounding host tissues or have issues associated with donor site morbidity. Tissue engineered scaffolds made from synthetic or natural materials are in development and avoid donor site morbidity issues, but often still do not provide the same mechanical function as natural cartilage. A natural, xenogenic, acellular composite osteochondral graft with the same composition and structure should have the same biomechanical function as natural cartilage. By removing the cells such grafts should be biocompatible. By retaining the subchondral bone, composite grafts should integrate well with the surrounding host tissues. No donor site is required and grafts should eventually be regenerated by host cells.
A natural decellularised composite bone-connective tissue-bone replacement or osteochondral tissue matrix that is immunologically inert and essentially devoid of any cells would offer immediate benefit to patients and the medical profession alike.
In particular, an “off the shelf” meniscal and bone replacement would address the social and economic needs of the ageing population with meniscal injuries enabling high levels of activity, capacity to work and quality of life. Similarly, a ligament replacement, osteochondral or repair implant would also be of significant value to sufferers of acute and chronic ligament and bone injuries alike. In particular a bone-patella tendon-bone implant would be of immense benefit to sufferers of anterior cruciate ligament injuries of the knee joint.